To measure the air quality in a city is a challenge. A large number of gaseous components need to be measured with high accuracy. The monitoring site needs to be representative for the city and the system has to be easy to maintain and calibrate. The OPSIS open-path monitoring systems provide a flexible solution, giving the average concentration for an area. A single OPSIS system can measure all relevant gaseous components set by the authorities, such as NO, NO2, SO2, O3, BTX, NH3.